The invention relates to heat exchangers. More specifically, the invention relates to oxidation protection of titanium-based heat exchangers.
Certain heat exchangers used in aircraft environmental control systems are exposed to temperatures exceeding 400xc2x0 C. These heat exchangers are typically made of stainless steel, which can withstand the high temperatures.
It would be desirable to use titanium heat exchangers instead of stainless steel heat exchangers. Titanium and its alloys have a light weight and can provide a weight reduction of up to 40% over comparable stainless steel heat exchangers. The weight reduction results in better fuel efficiency and lower aircraft operating costs.
However, titanium is not used for high temperature heat exchanger applications because the titanium exhibits a propensity to rapidly oxidize, (over a couple of hours), at the required operating temperatures. Oxidation of titanium results in a reduction in ductility and then strength, and a deterioration in structural integrity. Repeated thermal cycling at temperatures between ambient temperature and around 400xc2x0 C. (and higher) causes the titanium to crack. Cross contamination of fluids can occur and lead to life-threatening conditions.